Various electronic devices use electrical circuit boards or flexible flat cables to arrange, position, and mount electrical parts and connectors, and to transmit electrical signals to desired destination functional elements. Manufacturing of circuit boards is often conducted by laying signal transmission traces, through wiring technology, on a surface of a substrate. The commonly known circuit boards are generally classified as rigid circuit boards and flexible circuit boards or flat cables.
Taking a flexible flat cable that is commonly used in various electronic products, such as notebook computers, personal digital assistants, and mobile phones as an example, the flexible flat cable has such a structure that a number of conductors that are each covered by an insulation layer are arrange to juxtapose each other to form a flat structure that may be combined with an electrical connector or circuit soldering to realize transmission of electronic signals.
FIGS. 1 and 2 respectively show a schematic plan view and a cross-sectional view of a conventional circuit board. As shown in the drawings, a substrate 1 has a first surface 11 and a second surface 12. The substrate 1 extends in an extension direction to form a free end on which a plurality of conductive contacts 13 is formed. The substrate 1 may be inserted, with the free end thereof, into an insertion slot 15 formed on a circuit substrate 14 to allow the conductive contacts 13 to electrically engage corresponding contacts formed in the insertion slot 15.
The first surface 11 of the substrate 1 forms a plurality of parallel and spaced signal transmission lines 2 that extends a predetermined length. Adjacent signal transmission lines 2 on the first surface 11 of the substrate 1 are spaced from each other by a predetermined distance d, whereby the first surface 11 of the substrate 1 forms a plurality of wiring sections A1 that are respectively covered by the signal transmission lines 2 and a plurality of spacing zones A1 that are not covered by the signal transmission lines 2. A covering insulation layer 3 is formed on the first surface 11 of the substrate 1 to cover the surface of each signal transmission line 2 and each spacing section A2. A conductive shielding layer 4 is further formed to cover a surface of the covering insulation layer 3. The conductive shielding layer is often connected to system grounding to improve shielding performance and also to serve as a grounding surface for electrical impedance.
Both rigid circuit board and flexible circuit board are structured by laying a number of signal transmission lines 2 on a surface of a substrate 1. The signal transmission lines 2, although having a very small thickness, make the surface of the covering insulation layer 3 that is formed on the first surface 11 of the substrate 1 showing a corrugated wavy configuration and is thus irregular. Consequently, the conductive shielding layer 4 that is formed on the surface of the covering insulation layer 3 shows an irregular surface and wavy configuration as shown in FIG. 2.
Due to such a structural arrangement, the electrical impedance of each signal transmission line of the circuit board may be varied, making it hard to control the precision of the electrical impedance.
In a practical application, such a circuit board may cause certain problems, including poor impedance matching, signal reflection, radiation of electromagnetic wave, loss or missing of signal in signal receiving, and deformation of signal waveform. These problems cause sever influence on the circuit boards that are used in high precision electronic equipments.
This is especially problematic for electronic devices that are operated with high working frequency, such as notebook computers. Due to the high working frequency, the requirement for precision of impedance is severe. The conventional circuit boards that are manufactured with the conventional technology do not meet the needs of the industry. For flexible circuit board or flat cable, due to being made thin and flexible, the insulation layer is often formed of a coverlay and this makes corrugated configuration more severe and it is even harder to realize precision of electrical impedance.